Under the Hood

Troubleshooting the ignition system

Mark Lemay

Problems in this system may prevent the vehicle from starting, may cause the vehicle to run poorly while driving or may cause the vehicle to misfire. According to the vehicles age and model, the ignition system may include the ignition switch, distributor, spark plugs and wires, ignition coil or coils plus crankshaft and camshaft sensors.

A visual inspection, scanner, and an oscilloscope and a spark tester are the tools required to properly diagnose these systems.

When dealing with ignition problems, there are 3 areas of the systems that will cause problems: A) The voltage supply circuit, B) The output components which include the coil, spark plugs, ignition wires and distributor components if equipped, C) The switching circuit that turns the coil on and off. When diagnosing ignition problems, you must begin by determining which part of the system is the problem area. In all cases, tests should be performed with the vehicle running or cranking (if the vehicle will not start). Test points listed can be found in Figure 1.

Voltage test

Start your testing by hooking up a lab scope to test point F. This will test the supply voltage and the switching of the coil. With the engine cranking or running, there should be battery voltage at this point when the coil is off and the voltage should drop to near ground when the coil is turned on as shown in Figure 2. If the voltage to the coil is too low, then testing back through test points A through E should reveal the source of the problem. If the voltage is not dropping then testing through test points E and F should identify the problem area. If the circuit is good up to the module or PCM then the CKP and CMP sensors should be tested.

Coil test

If the coil is being properly switched and the supply voltage is good, then the coil output should be tested. To do this an adjustable spark tester should be used. For modern ignition systems the tester should be set at minimum of 40 KV or to about ¾ of an inch. Crank or start the engine and observe the spark tester. The coil should provide a spark that can jump the gap. If there is no spark then the coil, ignition wires or distributor components are faulty.


PCM and sensors test

Finally test how well the PCM is controlling the timing of the coil circuit. To do this, take a scope reading at test point F. The distance between coil firings should be consistent as shown in Fig 2. If the readings are not consistent, scan the PCM data stream and make sure the CKP data is correct. In Figure 3 we can see that the RPM data shows a fluctuation in RPM while the MAF sensor shows no change indicating that the RPM reading is false. This would lead to testing the CKP and CMP sensors to determine the cause of the failure. If the CKP and CMP sensors test good then the PCM or Ignition Module are faulty.

Following the testing methods described, you should be able to quickly identify and repair the cause of ignition system related failures.